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I have read that big optical zooms of let's say 10X do not produce excellent image quality. Is this true even nowadays, as there have been many technological improvements in this area?

For example, is 28-300 mm a good telephoto lens, despite the big optical zoom?

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    Check out the aperture range as well. A large zooming range often comes at a cost in aperture, making it harder to get good results in modest lighting conditions. – user27208 Apr 3 '14 at 15:51
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A greater zoom range means a more complex design at greatly increased cost. There are some excellent lenses make for broadcast with incredible zoom ranges, like the Fujinon 8-832mm (yes that's not a typo!), but you don't want to know the price.

Designing a lens with a larger zoom range at a lower price does lead to compromises on quality.

Finally lens technology moves very slowly compared to say camera sensors or processing chips, the nature of the game means you can't just take the same pixel design and shrink it for a quick win in performance. Advances come from better design tools more experience and improvements in manufacturing/quality control, all of which are gradual.

There have been advances recently but that's being driven by photographers becoming more demanding in terms of resolution, and those advances come at a price - as lenses are replaced with new designs the price is almost always higher.

Now you can get a "good" 28-300mm telephoto lens from Canon, but it's £2000, and not as good optically as the £1000 300mm f/4, which is a prime lens with no zoom. Canon could make a 28-300mm lens that was as good or better than the 300 f/4 prime, but it would be ten times the price.

Another good example is the Canon 70-200 f/2.8 zoom and the Canon 200 f/2.8 prime. The zoom is actually a little bit better than the prime at 200mm. But it's 3x the price and has a modest 3x zoom range. The wider the zoom range the more you'll have to pay to match the performance of a prime or smaller zoom range lens.

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    @Morpho: yes to all of that. A fixed focal length lens will usually be cheaper and have superior optical quality than a zoom lens at one end of its range, simply because it requires fewer elements and can be fully optimized for a specific focal length. – Michael Borgwardt Apr 3 '14 at 9:27
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    @Morpho: Lens design (and choice) is all about tradeoffs. You can have a cheap and good lens if you reduce complexity by foregoing things like zoom, image stabilization, AF... But price is often influenced by a lot of non-technical things. For example, a "premium" brand will never sell a lens cheaply even if it's cheap to make, simply because it would not fit their brand image. And anything that is sold in large numbers can be made cheaper due to economics of scale. – Michael Borgwardt Apr 3 '14 at 9:38
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    @Morpho The 300 f/4 is a prime (non-zoom) lens, which is easier to design/build, and can produce quite good image quality at the £1000 price point. You could make a 28-300mm lens that was as good or better than the Canon 300 f/4, but it would be ten times the price. As it is, the Canon 28-300mm is twice the price, and not as good. In general if you want better you have to trade the convenience of a zoom, or pay more for your lens. – Matt Grum Apr 3 '14 at 9:41
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    "Canon could make a 28-300mm lens that was as good or better than the 300 f/4 prime, but it would be ten times the price" and 10 times the weight... – jwenting Apr 3 '14 at 10:42
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    The Fujinon XA101x8.9BESM/PF, an 8.9-900mm broadcast zoom lens, can be had for only $205,659.95 USD. I wasn't even aware lenses could be that expensive! – nneonneo Apr 4 '14 at 7:39
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Yes, there have been a ton of advances in image quality over the years... HOWEVER... those improvements don't only apply to lenses with a huge zoom range, but also apply to lenses with a short zoom range or no zoom at all (primes). While a modern 28-200 lens may still be better at some ranges than a cheap lens with a much shorter zoom range from 30 years ago, it doesn't hold a candle to a modern lens at a similar price with a shorter range.

This simply boils down to complexity and compromises. Prime lenses are so cheap specifically because they can tune the lens to one particular focal length and not have to worry about anything else. A zoom lens is FAR, FAR more complex and the wider the zoom range becomes, the more compromises have to be made and the more elements added to the lens. Each additional element causes further distortion and the wider range of situations each has to deal with limit how well it can handle any one situation.

It is simply not physically possible to produce a super-zoom lens that is of similar quality to another lens of similar price.

That said, not all lenses have similar price and an expensive super zoom will still beat some much, much cheaper lenses, just like really high end normal zoom lenses can beat some much, much cheaper primes. You are talking an order of magnitude though, so a $1500 to $1800 superzoom may only beat the quality of a $300 normal zoom and be around the quality of a $400 or $500 normal zoom. (Those numbers are just thrown out there to illustrate the point, they aren't based around actual results in any particular lens system.)

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  • Thank you, AJ! For this reason, I have to concentrate on particular lenses and do not take anything as a general truth. Right? – Morpho Apr 3 '14 at 14:41
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    @Morpho because of the range of quality and price in lenses you should always look at the details of the lenses, however for any given price, it is pretty much universally true that for a given price, the primes will have the highest quality, with zooms a distant second and super-zooms a distant third beyond the normal zooms. The one exception to this is that if you are comparing an APS-c lens to a FF lens, the APS-c lens will be much cheaper for a given level of image quality since it doesn't have to make as big of an image circle. – AJ Henderson Apr 3 '14 at 14:43
  • For example Canon 55-250mm (any of the versions) cannot be fit on a full frame camera. Is this the reason to be considered cheap? – Morpho Apr 3 '14 at 14:50
  • EF-s lenses are cheaper than EF lenses of similar quality because they are less complex to build, however many EF-s lenses also have inferior quality to their EF counterparts. You can't tell how much of the difference is from the lack of complexity and how much of it is from image quality without looking at samples and/or MTF data. – AJ Henderson Apr 3 '14 at 14:54

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